1. Field of the Invention
This invention relates to microencapsulating processes and the microcapsules produced therefrom.
2. The Prior Art
A method for the production of microcapsules containing oils using coacervation is disclosed in U.S. Pat. No. 2,800,457 (1957) to Green et al. The process described therein involves the coating of oil droplets with a liquid wall of gelatin/gum arabic colloidal material produced by coacervation. The liquid wall thus formed is hardened by treatment with formaldehyde. It is known that aqueous solutions of hydrophilic colloids such as gelatins or gun arabics can be caused to undergo coacervation by adding to said solutions various substances such as inorganic salts or oppositely charged colloids. The coacervate droplets are known to encase droplets of water-immiscible or slightly water-immiscible liquids and to gel and solidify upon cooling to room temperatures thereby forming liquid droplets encased in a gelatin membrane. After hardening to the gelatin membranes in the normal fashion (such as with formaldehyde) encapsulated suspensions of liquids are obtained which after removal of water could be transformed by known drying methods into free-flowing dry powders. Other known patents teaching coacervation-related processes include:
U.S. Pat. No. 3,244,640 (1966) to Studt et al. PA0 U.S. Pat. No. 3,539,465 (1970) to Heistand PA0 U.S. Pat. No. 3,567,650 (1971) to Balcan PA0 U.S. Pat. No. 3,594,326 (1971) to Himmel PA0 U.S. Pat. No. 3,141,792 (1964) to Lachman et al. PA0 U.S. Pat. No. 3,449,228 (1969) to Yurcheshen et al. PA0 U.S. Pat. No. 3,779,942 (1973) to Bolles PA0 U.S. Pat. No. 3,875,074 (1975) to Vassiliades et al. PA0 U.S. Pat. No. 3,016,308 (1962) to Macaulay PA0 U.S. Pat. No. 3,516,846 (1970) to Matson PA0 U.S. Pat. No. 3,594,328 (1971) to Schibler PA0 U.S. Pat. No. 3,607,775 (1971) to Yoshida et al. PA0 U.S. Pat. No. 3,660,304 (1972) to Matsukawa PA0 U.S. Pat. No. 3,669,899 (1972) to Vassiliades et al. PA0 U.S. Pat. No. 3,778,383 (1973) to Schibler et al.
Other patents teaching processes of producing oil containing microcapsules include: U.S. Pat. No. 3,432,327 (1969) to Masao Kan et al. teaching interfacial reaction of a water soluble substance and oil soluble substance to form a resinous microcapsule wall, and U.S. Pat. No. 3,779,941 (1973) to Powell teaching the interfacial reaction of water soluble and oil soluble polymers, and U.S. Pat. No. 3,016,308 (1962) to Macaulay teaching a method of producing microcapsules by spray drying an emulsion which contains a film-forming substance in the continuous phase.
All of the above described processes of microencapsulation suffer from one or more of the following disadvantages, particularly, if commercial production of the resulting microcapsules, for use in aqueous coating compositions, is desired. Either the process requires a series of complex steps including special apparatus and in most instances coacervation, or the microcapsules produced by the process cannot be conveniently used in aqueous coating compositions. In some of the above patented processes, most notably those involving interfacial polymerization, the wall thickness is limited because of the need for diffusion of at least one of the reactants through the microcapsule wall. The use of coacervation in a microencapsulation process suffers from other disadvantages in that coacervation results in the production of clusters and aggregates of microcapsules as opposed to the more desirable production of discrete individual microcapsules.
The process of this invention includes a number of advantages not found in prior art methods. The materials used are relatively inexpensive and are readily available in commercial quantities. The process requires no complex steps, simply emulsification of the desired droplet solution in the solution containing the wall forming compound and then temperature adjustment to initiate the reaction of the wall forming compound and to form the microcapsule wall. Because of the non-sticky nature of the wall forming compounds, microencapsulation of single droplets of oil is easily obtained, even at relatively high concentrations, as opposed to the clusters and aggregates generally obtained by coacervation. The wall thickness of the microcapsules is not limited as in some prior art processes, since the microcapsule wall is formed by external polymerization of the wall forming compound and by the interaction of the wall forming compound with the cross-linking agent. The wall thickness can be controlled by the oil droplet size and relative amounts of wall forming compound and oil to be microencapsulated.
By the process of this invention, dispersions of approximately 40% microcapsules have been produced as compared to the more usual 25% obtained by prior art methods involving coacervation. Thus, the dispersion of microencapsulation can be used without dewatering in coating compositions where a high solids content is desirable.
Other patents considered relevant although inferior to the product and processes of this disclosure are:
All of the above patents and disclosures relate to microcapsules in general. The invention disclosed herein relates, specifically, to urea-formaldehyde-resorcinol microcapsules and more particularly to the process or the formation or manufacture of said urea-formaldehyde-resorcinol capsules. It should be noted that when referring to applicants' process the phrases "urea-formaldehyde capsules" and "urea-formaldehyde-resorcinol capsules" are used interchangeably. Such interchangeable use is not possible when describing the various prior art patents and applications. While the use of urea-formaldehyde-resorcinol capsules per se is well known in the art as demonstrated by the prior art cited hereinbelow, it has been shown that urea-formaldehyde-resorcinol microcapsules are capable of being produced by a variety of methods. Many advantages are enjoyed by each of the methods which are disclosed by the prior art; however, each also suffers from commercial disadvantages. The process disclosed herein and enjoyed by applicants is commercially and economically superior to those processes disclosed by the prior art for the manufacture of urea-formaldehyde-resorcinol microcapsules.
In particular, most of the prior art relates to the use of various regulating agents, systems modifiers, catalysts and other chemical additives to the urea-formaldehyde-resorcinol process and the pH adjustment steps which are concomitant with such chemicals. In particular, attention is drawn to U.S. Pat. Nos. 4,001,140 (1977); 4,087,376 (1978); and 4,089,802 (1978), all of which were issued to Foris et al. and deal with the manufacture of urea formaldehyde microcapsules. The only distinction between these three patents is the particular "systems modifier" which is recited. In U.S. Pat. No. 4,001,140, this material is described as a "negatively charged, carboxyl-substituted, linear aliphatic hydrocarbon polyelectrolyte material." This is, in fact, the generic description for the systems modifier described in all three of these patents. Each patent contains a separate listing of the precise chemicals which are applicable and which fall under the generic description.
Additional prior art which deals in general with urea-formaldehyde microcapsules consists of Japanese Patent Publication No. 12518/63 and Japanese patent application No. 9079/79 (an unexamined Japanese patent application) as well as pending U.S. application Ser. No. 926,369 which was filed in July of 1978, now U.S. Pat. No. 4,251,396, issued on Feb. 17, ,1981.
Finally, other patents mentioning urea-formaldehyde and in some instances going into minor detail regarding urea-formaldehyde microcapsules include:
As can be seen from the above, there is substantial prior art dealing with the urea-formaldehyde microcapsules. However, it is applicant's opinion that all of the prior art mentioned hereinabove suffer from commercial and economic disadvantages which are not characteristic of applicant's improved urea-formaldehyde manufacturing process. In general, it can be stated with regard to some of the above encapsulation processes that the wall membrane of the microcapsules prepared suffer from thermal stability and strength properties when oily solutions are used as the internal phase as in the carbonless paper environment. In many instances, the carbonless paper manufactured using these microcapsules is subject to premature color formation and capsule leakage. In addition, other disadvantages which are sometimes characteristic of many capsular systems are evidenced by some of the prior art systems, namely, non-uniform capsule size; inability to form small microcapsules; inability to withstand storage and use conditions and lack of appropriate rupturability in the formation of carbonless papers. These and other disadvantages of the prior art are overcome by applicant's process.